The P{UAS-mCherry.mir-276a.sponge.V2} and P{UAS-mCherry.mir-276b.sponge.V2} constructs target mir-276a-3P and mir-276b-3P respectively. However, because the only difference between mir-276a-3P and mir-276b-3P is at position 10, and position 10 is in the sponge bulge (not complementary to the miRNA), both sponges target both miRNAs.
The lack of a Scer\GAL4tub.PU-mediated viability phenotype is a false negative (compared to independent viability data from miRNA deletion mutants).